1. Field of The Invention
The present invention relates to an apparatus and a method for reinforcing a jet pump riser for preventing occurrence of a weld line crack of a jet pump riser fixed by being welded to an inside of a reactor pressure vessel of a boiling water reactor for preventing development of the crack that has occurred and for maintaining a function at the time of rupture of the weld line.
2. Related Art
Jet pumps in a boiling water reactor are disposed at an annular portion (downcomer portion) between a core shroud of a reactor pressure vessel (RPV) and an RPV inner wall and coupled to a recirculation system to circulate a coolant to a rector core.
With reference to FIG. 10, a structure around the jet pump in the reactor pressure vessel 1 will be described schematically.
As shown in FIG. 10, the jet pump 1a includes a riser 10a, an inlet mixer 9, a diffuser 13, and the like. The riser 10a includes a riser pipe 10 provided perpendicularly to one end of a riser elbow 11 connected to a recirculation nozzle 8, and the riser pipe 10 is fixed to the riser elbow 11 by means of welding. Furthermore, a transition piece 14 is fixed by means of welding to an upper end portion of the riser elbow 11. The other end of the riser elbow 11 is fixed by means of welding to a thermal sleeve 12 of the RPV recirculation inlet nozzle 8.
The riser elbow 11 of the inlet mixer 9 is placed on and fixed by a hold-down bolt to an upper surface of the riser 10a. The inlet mixer 9 is assembled by welding the riser elbow 11, a mixing nozzle 15 and a nozzle throat 16, and a lower end of the nozzle throat 16 is inserted into an upper end of the diffuser 13.
The diffuser 13 is assembled by welding an annular member to a lower portion of a flared cylinder and a lower end of the diffuser 13 is fixed by means of welding to a shroud support plate 5.
Driving fluid with its pressure increased by a recirculating pump, not shown, provided outside the reactor pressure vessel 1 is supplied from the recirculation inlet nozzle 8 into the jet pump 1a and forced to circulate from the riser 10a to the reactor core via the inlet mixer 9 and the diffuser 13.
In the reactor having the above structure, fluid oscillation occurs due to a flow of a large quantity of cooling water sent in from the recirculating pump, and stress repeatedly acts on the jet pump 1a. 
Further, during the operation, stress due to a pressure difference between the inside and outside and a large quantity of heat at a reactor core portion occur, and a thermal stress acts on the jet pump.
Because of the stress applied repeatedly due to the fluid oscillation, the pressure difference and the thermal stress, fatigue cracks may occur at the welded portion between the riser elbow 11 and the thermal sleeve 12, and the welded portion between the riser elbow 11 and the riser pipe 10.
Further, cracks may be produced by the stress corrosion crack that occurs in a sensitized region of a welding heat-affected zone.
By continuing the operation of the reactor, the crack may develop to rupture the welded portion in extreme cases. As a result, it may be difficult to maintain a function of sending the fluid (water) into the reactor core of the jet pump, which provides an inconvenience from a viewpoint of safely maintaining operation of the reactor.
As prior art, there are disclosed a method in which vicinities of welded portions of the riser elbow 11, the riser pipe 10, and the thermal sleeve 12 are clamped with clamps and both the clamps are coupled, for example (see Japanese Patent Application Laid-open No. 10-206587: Patent Publication 1), a method in which a vicinity of the welded portion between the riser elbow 11 and the thermal sleeve 12 is clamped, and reaction force of the driving fluid flowing inside from the recirculation inlet nozzle 8 is received by a core shroud 7 (see Japanese Patent Application Laid-open No. 10-186080: Patent Publication 2, for example), and the like.
In the technique in the above-described Patent Publication 1, the clamps are linearly fastened to each other and, if a weld line ruptures and twist force is applied, for example, the ruptured portion opens and it may become impossible to maintain the function of the jet pump 1a. 
In the technique in the Patent Publication 2, the riser elbow 11 is restrained by the core shroud 7 in a horizontal direction, and there is also proposed a method of supporting the riser elbow 11 on the diffuser 13 depending on the status of implementation. These methods may complicate behavior of oscillation applied to the jet pump 1a and also exert undesirable influence on the welded portion between the diffuser 13 and the shroud support plate 5.